Have you considered getting into embedded development? You tend to be dealing with much lower level code and closer to the hardware. It's fun :)
I will probably move to embedded soon. Problem is I have lot of experience with iOS but not that much experience with embedded, I'm willing to get a pay cut just to get out of iOS but there's a limit of how low I can go.
There's also a lot more iOS jobs than embedded jobs!
edit: Other than C, I'm very curious about rust and nim-lang. I do not know C++ though. Hope that won't be a problem.
I've found that it entirely depends what level of embedded development you want to enter - for instance I mainly work with C++ and MSQL databases on our platform - because our main device runs linux this makes sense. But other related products that we sell run on bare metal and are written in pure C.
Having a strong knowledge of C will most certainly get you far in this field, but perhaps learning the ins and outs of C++ wouldn't be such a bad idea if you have the spare time - the more niches you can fit in the easier it will be to pick up that job you really want.